An electrically insulating substrate material is used for an IC substrate, a sensor substrate, a solar cell substrate, an electrode substrate or the like, and this material is essential in the electronic and electric industry.
With the diversification of usage in the future, the electrically insulating substrate material is required to have heat resistance, hardness and strength. Furthermore, the structure of a product is complicated while reduction in the size and weight is required, and therefore, a thin material, a lightweight material and a material with good processability are demanded. A stainless steel foil coated with an insulating film is considered to be capable of responding to such needs.
A material produced by forming a resin layer on a substrate has been conventionally known but this material has a problem in the heat resistance, hardness and the strength of the resin layer. Also, a material produced by film-forming a ceramic such as SiO2 or SiN on a stainless steel sheet by plasma CVD (see, Japanese Unexamined Patent Publication (Kokai) No. 6-306611) or a material produced by film-forming a ceramic such as TiN, TiC or TiOx on a stainless steel foil by ion plating (see, Japanese Unexamined Patent Publication (Kokai) No. 6-322516) have been disclosed. However, in the case where the coated film is a ceramic film, that is, an inorganic material, the flexibility is insufficient despite excellence in the heat resistance, hardness and strength and when this film is formed on a substrate having flexibility, cracking is readily generated in the ceramic film on the substrate and there not only arises a problem such as failure in maintaining the insulating property but even separation of the film, from the stainless steel foil, occurs due to stress produced at the crack-generating site.
Furthermore, a method of coating a polysilazane-containing coating solution on a resin, stainless steel or glass substrate to form a silica film on the substrate has been disclosed (see, Japanese Unexamined Patent Publication (Kokai) No. 2001-111076) but in the case of stainless steel or glass substrate, the substrate itself is poor in processability.
In addition, for example, Japanese Unexamined Patent Publication (Kokai) No. 61-003474 discloses the importance of flatness of the substrate by stating that in the case of a metal substrate for a solar cell, if an inclusion in a size of 1.0 μm or more is present, the metal substrate is short-circuited with the electrode provided as an outermost surface and this causes a fatal defect or the like in the solar cell. However, in the film-forming method using a dry process, the film deposits following the surface roughness and therefore, satisfactory flatness may not be obtained on the material surface.
Accordingly, a first object of the present invention is to solve the problems in those conventional techniques and provide a stainless steel foil coated with an inorganic-organic hybrid film excellent in the heat resistance, processability, flatness, flexibility and insulating property.
Also, along with recent reduction in the size and weight of a final product, the electrically insulating substrate material is required to be thin and lightweight and have good processability.
For example, in the production of a thin-film solar cell having flexibility, a resin such as polyimide is coated on a supporting substrate such as glass and then cured, respective layers of an amorphous solar cell are stacked thereon, and the stack is dipped in water to separate layers of the amorphous solar cell together with the resin from the supporting substrate. In the case of polyimide generally employed, if the film is formed by allowing the imidation reaction to completely proceed, the film can be hardly separated from the supporting substrate. When the film is heat-treated at a low temperature, the separation is facilitated but, as insufficient imidation results, an out-gas may be generated at the formation of an amorphous silicon and this may cause deterioration in the quality of the amorphous silicon film, an elevation of water absorptivity or a decrease in reliability. In this way, the separation type has dual problems of separability and reliability and it is required, in practice, to solve these problems.
In another type of a flexible thin-film solar cell, an organic resin coating material such as polyimide coating material having high insulating property is coated on a metal foil (see, Japanese Unexamined Patent Publication (Kokai) No. 2001-127323). However, many of organic resins deteriorate in the heat treatment at 200 to 350° C. during formation of a solar battery cell. Even in the case of an organic resin having high heat resistance such as polyimide, the thermal expansion coefficient is large as compared with an inorganic material and therefore, cracks or the like are generated in the formed solar battery cell, on cooling, and the power generation efficiency decreases. Therefore, for a silicon-based thin-film solar cell with flexibility, a metal foil coated with an insulating material having high heat resistance and a small thermal expansion coefficient is required. The insulating property is on the MΩ·cm2 order in the case of a solar cell substrate or the like.
Furthermore, also in a solar cell using a compound semiconductor thin-film system known to have high power generation efficiency, realization of a large area and a low cost is required and studies are being made to form a cell on a stainless steel foil suitable for the roll-to-roll system (see, Data of The New Energy and Industrial Technology Development Organization, pp. 5-6 (Jun. 7, 2002)). For connecting in series respective unit cells by collecting solar battery cells, an insulating film is formed on a stainless foil, a lower electrode is film-formed thereon, and a semiconductor film such as Cu—In—Ga—Se is stacked. The property required, of the insulating film, is high heat resistance, because it is necessary to perform vapor deposition by elevating the substrate temperature to 500 to 600° C. at the production of a semiconductor film or apply a heat treatment at 500 to 600° C. after sputtering. A material produced by film-forming a ceramic such as SiO2 or SiN on a stainless steel by plasma CVD (see, Japanese Unexamined Patent Publication (Kokai) No. 6-306611) or a material produced by film-forming a ceramic such as TIN, TiC or TiOx on a stainless steel foil by ion plating (see, Japanese Unexamined Patent Publication (Kokai) No. 6-322516) is disclosed. However, in the case where the insulating film is a ceramic film, that is, an inorganic material, the flexibility is insufficient despite excellent heat resistance and when this film is formed on a substrate having flexibility, cracking is readily generated in the ceramic film on the substrate and not only does there arise a problem such as failure in maintaining the insulating property but also separation of the film, from the stainless steel foil, occurs due to stress produced at the crack-generating site. Also, in the film-forming method such as sol-gel method, as cracking is generated when the film thickness is increased, a film thickness large enough to ensure the insulating property of MΩ·cm2, required of the solar cell substrate, is not obtained in many cases. On the other hand, an insulating film of an organic resin system such as polyimide is not satisfactory in heat resistance. Furthermore, an Mo metal is usually used as the lower electrode in a compound semiconductor-based solar cell, but there is a problem that the adhesion of Mo to an organic material having a large thermal expansion coefficient is extremely poor.
Accordingly, a second object of the present invention is to provide a stainless steel foil coated with a plurality of inorganic-organic hybrid films so as to satisfy all of heat resistance, insulating property on the order of 1 MΩ·cm2, and adhesion to a device layer.